In this R34 application (PAR-14-179: Clinical Trial Planning Grant for Interventions and Services to Improve Treatment and Prevention of HIV/AIDS), we propose to develop and pilot test an intervention to improve the science of HIV self-testing (HST) as a public health strategy to increase HIV testing among populations with low testing rates. HST offers a compelling strategy for achieving higher levels of HIV testing, due to the reduced barriers associated with seeking traditional forms of clinic-based HIV testing. However, HST scale-up alone is insufficient as a public health strategy: individuals who receive an HST negative result require motivation to engage in repeat testing and encouragement to reduce their risk behaviors. Thus, we propose a combination strategy that incorporates HST with mobile application- (app-) based HIV risk reduction and behavior change motivation messages. The proposal research will take place in China, where our prior research has demonstrated low rates of testing, high behavioral risk, and barriers to clinical HIV prevention services among men who have sex with men (MSM). The research will leverage the broad general access to mobile phone and app-based electronic messaging technology in the Chinese population to develop a test a combined HST plus mHealth app-based messaging strategy for MSM. We will use a three-phase research approach to address our specific aims. First, we will conduct formative qualitative research to understand the facilitators and barriers to HIV self-testing (HST) among MSM in China. Second, based on qualitative findings, we will develop a library of brief messages to motivate risk reduction and behavior change, and we will use cognitive interviewing techniques and an open pilot to refine and finalize the app-based messaging content and implementation strategy. Third, we will pilot test the combination of HST plus mobile app-based risk reduction messaging compared to HST alone in a sample of high-risk MSM, with respect to repeat HST and changes in sexual risk behaviors in the next 6 months. The proposed study is innovative because it is the first known attempt to develop a combination approach to HST that leverages advances in mobile technology as a platform for repeat testing and motivating behavior change. We will use findings to support a subsequent application to test the effectiveness of the combined HST plus app-based messaging intervention on sexual risk behavior and HIV retesting.